The Palaeontological Association  Reg. Charity No. 276369 Site Accessibility  
The Palaeontogical Association Logo

Loading Picture (.gif) Loading Page ...

PalAss Home > PalAss Newsletter > Cladistics for Palaeontologists - Introduction
Cladistics for Palaeontologists - Introduction
Page: 1/5

Next Page (2/5)

Cladistics was introduced by the German entomologist Willi Hennig, who put forward his ideas in 1950. He wrote in his native language, so these were completely ignored until 1966 when an English translation of a manuscript was published under the title “Phylogenetic Systematics” (Hennig 1966). It is not an easy book to read but fortunately many others have been written that have both fl eshed out and distorted his ideas. Hennig’s most important contribution was to offer a precise defi nition of biological relationship and to suggest how that relationship might be discovered.

Taxon and character relationship

Hennig’s concept of relationship is illustrated in Figure 1. Considering three taxa, then the salmon and the lizard are more closely related to each other than either is to the shark. This is so because the salmon and the lizard share a common ancestor, ‘x’, which lived at time t2 and which is not shared with the shark or any other taxon. Similarly the shark is more closely related to a group ‘salmon+lizard’ because the shark, salmon and lizard together share a unique common ancestor – ‘y’, which lived at an earlier time t1. The salmon and lizard are called sister-groups; the shark is the sister-group of the combined group salmon+lizard. By extension, the lamprey is the sister-group of shark+salmon+lizard. The aim of cladistic analysis is to discover this sister-group hierarchy, and express the results in branching diagrams. These diagrams are called cladograms, a reference to the fact that they purport to express the genealogical units or clades (the word ‘Cladistics’ was, ironically, coined by Ernst Mayr – a life-long opponent of cladistic classification). The aim of cladistics is to search for the sister-group, and the concept of two taxa being more closely related to each other than either is to a third (the three-taxon statement) is fundamental to cladistics.

Figure 1. Hennig’s concept of relationships among taxa A – D

Figure 1. Hennig’s concept of relationships among taxa A – D. See text for discussion.

Sister-groups are discovered by identifying characters (or character states) that are uniquely shared by two of the three groups under consideration. But not just any characters (or character states – we will deal with the relation between character and character state in the next article).Hennig made a distinction between two types of characters (or character states) and this distinction depended on where they occurred in the phylogenetic history of a particular group. The character or the state of the character which occurs in the ancestral morphotype he called “plesiomorphic” (near to the ancestral morphology), and the derived character, or the derived state, he called “apomorphic” (away from the ancestral morphology). Here, it is only necessary to emphasise that the terms apomorphic and plesiomorphic are relative terms – relative to a particular systematic problem. In Figure 2A character state “a” is plesiomorphic and “a prime” is apomorphic. State “a prime” is presumed to have been present in the ancestral morphotype which gave rise to taxa B and C. The presence of character “a prime” – the apomorphic state – in taxa B and C is evidence of their immediate common ancestry and their sister-group relationship. “a prime” is a shared apomorphy or a synapomorphy suggesting that taxa B and C are more closely related to each other than either is to A. In Figure 2B “a prime” is apomorphic with respect to “a” but it is plesiomorphic with respect to “a double prime”. So, just as the relationship of taxa is relative, so is the relationship of characters (or character states).

Figure 2. Hennig’s ideas of relationships between character states.
Figure 2. Hennig’s ideas of relationships between character states. See text for discussion.

Hennig thought that you could decide which was the apomorphic state and which was plesiomorphic before you did the analysis. He had several criteria for this of which stratigraphic order was the most relevant to us – the state of a character that occurs earlier in the fossil record is to be regarded as the plesiomorphic state. This did not go down very well with neontologists, nor with many palaeontologists, because it relied on the faithfulness of the fossil record to document the truth. Today, there are two criteria that are used: the outgroup and ontogenetic sequence, both of which we will explore in the next article.

Hennig introduced a third state that he called autapomorphic. This is the state that occurs in only one of the taxa under consideration. And once again, autapomorphic characters in one analysis may be synapomorphies in another.

A real example is given in Figure 3. In Figure 3 characters numbered 3 and 4 are synapomorphies suggesting that the lizard and the salmon shared a unique common ancestor ‘Z’. It suggests that characters 3 and 4 arose in ancestor ‘Z’ and were inherited by the salmon and the lizard. Shared primitive characters (symplesiomorphies) are characters inherited from a more remote ancestry and are irrelevant to the problem of relationship of the lizard and the salmon. For example, the shared possession of characters 1 and 2 in the salmon and lizard would not imply that they shared a unique common ancestor because these attributes are also found in the shark. Characters 1 and 2 may be useful at a more inclusive hierarchical level to suggest common ancestry at ‘Y’. With respect to the three-taxon problem (shark, salmon and lizard) then characters 1 and 2 are symplesiomorphies and they suggest nothing other than that the shark, salmon and lizard are a group. Similarly, characters 5 – 9 and 10 – 12 are autapomorphies and irrelevant to discovering relationships since they are each found in only one of the taxa. Sister-groups are discovered by identifying shared derived apomorphic characters (synapomorphies) inferred to have originated in the latest common ancestor and shared by descendants. These synapomorphies can be thought of as evolutionary homologies: that is, as structures inherited from the immediate common ancestor.

Figure 3. An example of a phylogeny showing characters by which taxa are recognised.

Figure 3. An example of a phylogeny showing characters by which taxa are recognised. Characters 1 – 4 are synapomorphies, 5 – 12 are autapomorphies and 13 is an attribute seen in the salmon and the shark. See text for discussion.

Another way we can think of this is to ask the question “what groups are specified by what characters?” In Figure 3 given four taxa, of (initially) unknown interrelationships, then characters 1 and 2 suggest a group Shark + Salmon + Lizard. Characters 3 and 4 suggest a group Salmon + Lizard. But characters 1, 2, 3 and 4, suggest two nested groups, one more inclusive than the other ((Shark (Salmon, Lizard)).

Next Page (2/5)

Created by Alan R.T. Spencer on the 2007-02-09. (Version 2.0)